1. Field of the Invention
The present invention relates to a piezoelectric film containing lead zirconate titanate, a piezoelectric element using the piezoelectric film, and a liquid discharge apparatus.
2. Description of the Related Art
An actuator including an ink jet recording head is provided with a piezoelectric body having a piezoelectric property that expands and contracts with variation in applied electric field intensity, and a piezoelectric element provided with an electrode that applies an electric field to the piezoelectric body.
In recent years, in order to meet the demand for miniaturization of an apparatus, miniaturization of an actuator in cooperation with a semiconductor process technology such as a microelectromechanical systems (MEMS) technology has proceeded. In the semiconductor process technology, high-precision processing using film formation or photolithography becomes possible. Therefore, there has been actively conducted research on thinning of a piezoelectric body in an actuator.
As a piezoelectric material having high piezoelectric properties, a lead zirconate titanate (PZT)-based perovskite oxide has been widely used due to its performance. It is known that when a PZT-based perovskite oxide piezoelectric film has a morphotropic phase boundary (MPB) composition in which Zr:Ti is near 52:48, the piezoelectric constant and the electromechanical coupling coefficient thereof become maximum, which is appropriate for actuator applications.
In JP2012-99636A, it is described that in a piezoelectric element provided with a piezoelectric thin film having a laminate of a lead titanate layer and a lead zirconate layer, which have columnar structures, the compositions of lead titanate and lead zirconate in the piezoelectric thin film are caused to be MPB compositions, thereby improving piezoelectric properties.
On the other hand, as a technique for improving piezoelectric properties using a method other than that for the formation of the MPB compositions, doping a PZT-based piezoelectric film with various donor ions having higher valences than those of substituted ions is known. Since the ionic valence of Zr and Ti in B-site is 4, as donor ions that substitute for B-site elements, B-site elements having an ionic valence of 5 or higher, such as V, Nb, Ta, Sb, Mo, and W have been used. The amount of the donor ions that substitute for the B-site elements is very small. Specifically, in a case of Nb doping, the doping amount is about 0.2 to 0.025 mol % (refer to J. Am. Ceram. Soc, 84 (2001) 902 and Phys. Rev. Let, 83 (1999) 1347). This is because the crystallization temperature rises to 800° C. or higher in a case of doping with a large amount of Nb.
Here, WO2012/124409A discloses a ferroelectric thin film formed by depositing a dielectric material having a perovskite structure on a base body, in which the dielectric material is composed of a composite oxide in which metal materials other than Pb, Zr, and Ti as additives are mixed in PZT, the thin film has layers with different Zr/Ti ratios, a first layer having a small Zr mixing ratio is provided on the base body, and a second layer having a large Zr mixing ratio is provided on the first layer. In this configuration, the quality of crystallinity varies depending on the mixing concentration of the additives, and the crystallinity is improved in a case where the Zr mixing ratio is smaller than a predetermined ratio even in the dielectric material with varying piezoelectric properties, and good piezoelectric properties are exhibited in a case where the Zr mixing ratio increases to about the predetermined ratio. Therefore, the ferroelectric thin film which exhibits predetermined piezoelectric properties with a predetermined thickness can be formed with good crystallinity by combining the first layer having a mixing ratio for good crystallinity with the second layer having a mixing ratio for high piezoelectric properties. That is, a ferroelectric thin film which exhibits high piezoelectric properties is obtained by forming a thin film of the ferroelectric with good crystallinity on the base body in which a lower electrode layer is formed on a substrate. However, there is a need to form the first layer and the second layer on the base body using target materials with different Zr/Ti ratios in order to change the Zr mixing ratio, and thus the production is complex.
In JP2006-287254A, as a method of obtaining high piezoelectric properties and suppressing cracking during film formation, the amount of added metals doping PZT is specified by performing sputtering in an atmosphere without oxygen and thereafter performing a heat treatment process. However, in the method of JP2006-287254A, a heat treatment is necessary later, and it is difficult to control the amount of Pb because Pb evaporates at this time. Furthermore, the specified amount of added metals cannot cause further improved properties.